I ran into this problem also I fixed it by searching through the web config file for System.Web.Extensions, Version=1.0.61025.0 and replaced it with System.Web.Extensions, Version=3.5.0.0

I believe there are somewhere in the region of 10 places in the web.config where System Web Extensions is explicitly referenced, You may also wish to delete the old version from the bin folder of your install.

Hope that Helps

John

Alex Tom wrote   Hi John, I think that is not the right solution, just replacing the Versions in the web.config. I tried this before and yes my module is working with System.Web.Extensions, Version=3.5.0.0. But now the original Modules like "add new page" or "portal settings" are not working any longer. I think they need the old System.Web.Extensions, Version=1.0.61025.0   I think you can do this when you develop your module and you recompile the hole dnn. Then everything works fine, but what can I do with a productive installed dnn 4.8.4 version?   Best regards, Alex

As I said before you need to upgrade your site to work with the 3.5 libraries. You cannot use older libraries AND newer libraries, cause there is a namespace collision.